Adapting short-wavelength LED&#39;s for polychromatic, broadband, or “white” emission

ABSTRACT

An adapted LED is provided comprising a short-wavelength LED and a re-emitting semiconductor construction, wherein the re-emitting semiconductor construction comprises at least one potential well not located within a pn junction. The potential well(s) are typically quantum well(s). The adapted LED may be a white or near-white light LED. The re-emitting semiconductor construction may additionally comprise absorbing layers surrounding or closely or immediately adjacent to the potential well(s). In addition, graphic display devices and illumination devices comprising the adapted LED according to the present invention are provided.

FIELD OF THE INVENTION

This invention relates to adaptation of short-wavelength LED's to emitpolychromatic or broadband light, which may appear as white ornear-white light, by addition of a re-emitting semiconductorconstruction to down-convert a portion of the emitted light to longerwavelengths.

BACKGROUND OF THE INVENTION

Light emitting diodes (LED's) are solid-state semiconductor deviceswhich emit light when an electrical current is passed between anode andcathode. Conventional LED's contain a single pn junction. The pnjunction may include an intermediate undoped region; this type of pnjunction may also be called a pin junction. Like non-light emittingsemiconductor diodes, conventional LED's pass an electrical current muchmore readily in one direction, i.e., in the direction where electronsare moving from the n-region to the p-region. When a current passes inthe “forward” direction through the LED, electrons from the n-regionrecombine with holes from the p-region, generating photons of light. Thelight emitted by a conventional LED is monochromatic in appearance; thatis, it is generated in a single narrow band of wavelengths. Thewavelength of the emitted light corresponds to the energy associatedwith electron-hole pair recombination. In the simplest case, that energyis approximately the band gap energy of the semiconductor in which therecombination occurs.

Conventional LED's may additionally contain one or more quantum wells atthe pn junction which capture high concentrations of both electrons andholes, thereby enhancing light-producing recombination. Severalinvestigators have attempted to produce an LED device which emits whitelight, or light which appears white to the 3-color perception of thehuman eye.

Some investigators report the purported design or manufacture of LED'shaving multiple quantum wells within the pn junction, where the multiplequantum wells are intended to emit light at different wavelengths. Thefollowing references may be relevant to such a technology: U.S. Pat. No.5,851,905; U.S. Pat. No. 6,303,404; U.S. Pat. No. 6,504,171; U.S. Pat.No. 6,734,467; Damilano et al., Monolithic White Light Emitting DiodesBased on InGaN/GaN Multiple-Quantum Wells, Jpn. J. Appl. Phys. Vol. 40(2001) pp. L918-L920; Yamada et al., Phosphor FreeHigh-Luminous-Efficiency White Light-Emitting Diodes Composed of InGaNMulti-Quantum Well, Jpn. J. Appl. Phys. Vol. 41 (2002) pp. L246-L248;Dalmasso et al., Injection Dependence of the Electroluminescence Spectraof Phosphor Free GaN-Based White Light Emitting Diodes, phys. stat. sol.(a) 192, No. 1, 139-143 (2003).

Some investigators report the purported design or manufacture of LEDdevices which combine two conventional LED's, intended to independentlyemit light at different wavelengths, in a single device. The followingreferences may be relevant to such a technology: U.S. Pat. No.5,851,905; U.S. Pat. No. 6,734,467; U.S. Pat. Pub. No. 2002/0041148 A1;U.S. Pat. Pub. No. 2002/0134989 A1; and Luo et al., Patternedthree-color ZnCdSe/ZnCdMgSe quantum-well structures for integratedfull-color and white light emitters, App. Phys. Letters, vol. 77, no.26, pp. 4259-4261 (2000).

Some investigators report the purported design or manufacture of LEDdevices which combine a conventional LED element with a chemicalphosphor, such as yttrium aluminum garnet (YAG), which is intended toabsorb a portion of the light emitted by the LED element and re-emitlight of a longer wavelength. U.S. Pat. No. 5,998,925 and U.S. Pat. No.6,734,467 may be relevant to such a technology.

Some investigators report the purported design or manufacture of LED'sgrown on a ZnSe substrate n-doped. with I, Al, Cl, Br, Ga or In so as tocreate fluorescing centers in the substrate, which are intended toabsorb a portion of the light emitted by the LED element and re-emitlight of a longer wavelength. U.S. Pat. App. No. 6,337,536 and JapanesePat. App. Pub. No. 2004-072047 may be relevant to such a technology.

SUMMARY OF THE INVENTION

Briefly, the present invention provides an adapted LED comprising ashort-wavelength LED and a re-emitting semiconductor construction,wherein the re-emitting semiconductor construction comprises at leastone potential well not located within a pn junction. The potentialwell(s) are typically quantum well(s). In one embodiment, there-emitting semiconductor construction additionally comprises anabsorbing layer closely or immediately adjacent to a potential well. Inone embodiment, the re-emitting semiconductor construction additionallycomprises at least one second potential well not located within a pnjunction having a second transition energy not equal to the transitionenergy of the first potential well. In one embodiment, theshort-wavelength LED is a UV LED. In one such embodiment, there-emitting semiconductor construction comprises at least one firstpotential well not located within a pn junction having a firsttransition energy corresponding to blue-wavelength light, at least onesecond potential well not located within a pn junction having a secondtransition energy corresponding to green-wavelength light, and at leastone third potential well not located within a pn junction having a thirdtransition energy corresponding to red-wavelength light. In oneembodiment, the short-wavelength LED is a visible light LED, typically agreen, blue or violet LED, more typically a green or blue LED, and mosttypically a blue LED. In one such embodiment, the re-emittingsemiconductor construction comprises at least one first potential wellnot located within a pn junction having a first transition energycorresponding to yellow- or green-wavelength light, more typicallygreen-wavelength light, and at least one second potential well notlocated within a pn junction having a second transition energycorresponding to orange- or red-wavelength light, more typicallyred-wavelength light.

In another aspect, the present invention provides a graphic displaydevice comprising the adapted LED according to the present invention.

In another aspect, the present invention provides an illumination devicecomprising the adapted LED according to the present invention.

In this application:

with regard to a stack of layers in a semiconductor device, “immediatelyadjacent” means next in sequence without intervening layers, “closelyadjacent” means next in sequence with one or a few intervening layers,and “surrounding” means both before and after in sequence;

“potential well” means a layer of semiconductor in a semiconductordevice which has a lower conduction band energy than surrounding layersor a higher valence band energy than surrounding layers, or both;

“quantum well” means a potential well which is sufficiently thin thatquantization effects raise the electron-hole pair transition energy inthe well, typically having a thickness of 100 mm or less;

“transition energy” means electron-hole recombination energy;

“lattice-matched” means, with reference to two crystalline materials,such as an epitaxial film on a substrate, that each material taken inisolation has a lattice constant, and that these lattice constants aresubstantially equal, typically not more than 0.2% different from eachother, more typically not more than 0.1% different from each other, andmost typically not more than 0.01% different from each other; and

“pseudomorphic” means, with reference to a first crystalline layer ofgiven thickness and a second crystalline layer, such as an epitaxialfilm and a substrate, that each layer taken in isolation has a latticeconstant, and that these lattice constants are sufficiently similar sothat the first layer, in the given thickness, can adopt the latticespacing of the second layer in the plane of the layer substantiallywithout misfit defects.

It should be understood that, for any embodiment of the presentinvention described herein comprising n-doped and p-doped semiconductorregions, a further embodiment should be considered as disclosed hereinwherein n doping is exchanged with p doping and vice-versa.

It should be understood that, where each of “potential well,” “firstpotential well,” “second potential well” and “third potential well” arerecited herein, a single potential well may be provided or multiplepotential wells, which typically share similar properties, may beprovided. Likewise, it should be understood that, where each of “quantumwell,” “first quantum well,” “second quantum well” and “third quantumwell” are recited herein, a single quantum well may be provided ormultiple quantum wells, which typically share similar properties, may beprovided.

It is an advantage of certain embodiments of the present invention toprovide an LED device capable of emitting polychromatic, white ornear-white light.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of an LED according to one embodiment ofthe present invention.

FIG. 2 is a flat-band diagram of conduction and valence bands ofsemiconductors in a construction according to one embodiment of thepresent invention. Layer thickness is not represented to scale.

FIG. 3 is a graph indicating lattice constant and band gap energy for avariety of II-VI binary compounds and alloys thereof.

FIG. 4 is a graph representing the spectrum of light that emits from adevice according to one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides an adapted LED comprising ashort-wavelength LED and a re-emitting semiconductor construction,wherein the re-emitting semiconductor construction comprises at leastone potential well not located within a pn junction. The potential wellsare typically quantum wells. In typical operation, the short-wavelengthLED emits photons in response to an electric current and the re-emittingsemiconductor construction emits photons in response to the absorptionof a portion of the photons emitted from the short-wavelength LED. Inone embodiment, the re-emitting semiconductor construction additionallycomprises an absorbing layer closely or immediately adjacent to thepotential well. Absorbing layers typically have a band gap energy whichis less than or equal to the energy of photons emitted by theshort-wavelength LED and greater than the transition energy of thepotential wells of the re-emitting semiconductor construction. Intypical operation the absorbing layers assist absorption of photonsemitted from the short-wavelength LED. In one embodiment, there-emitting semiconductor construction additionally comprises at leastone second potential well not located within a pn junction having asecond transition energy not equal to the transition energy of the firstpotential well. In one embodiment, the short-wavelength LED is a UV LED.In one such embodiment, the re-emitting semiconductor constructioncomprises at least one first potential well not located within a pnjunction having a first transition energy corresponding toblue-wavelength light, at least one second potential well not locatedwithin a pn junction having a second transition energy corresponding togreen-wavelength light, and at least one third potential well notlocated within a pn junction having a third transition energycorresponding to red-wavelength light. In one embodiment, theshort-wavelength LED is a visible light LED, typically a green, blue orviolet LED, more typically a green or blue LED, and most typically ablue LED. In one such embodiment, the re-emitting semiconductorconstruction comprises at least one first potential well not locatedwithin a pn junction having a first transition energy corresponding toyellow- or green-wavelength light, more typically green-wavelengthlight, and at least one second potential well not located within a pnjunction having a second transition energy corresponding to orange- orred-wavelength light, more typically red-wavelength light. There-emitting semiconductor construction may comprise additional potentialwells and additional absorbing layers.

The adapted LED according to the present invention may be composed ofany suitable semiconductors, including Group IV elements such as Si orGe (other than in light-emitting layers), III-V compounds such as InAs,AlAs, GaAs, InP, AlP, GaP, InSb, AlSb, GaSb, and alloys thereof, II-VIcompounds such as ZnSe, CdSe, BeSe, MgSe, ZnTe, CdTe, BeTe, MgTe, ZnS,CdS, BeS, MgS and alloys thereof, or alloys of any of the above. Whereappropriate, the semiconductors may be n-doped or p-doped by anysuitable method or by inclusion of any suitable dopant. In one typicalembodiment, the short wavelength LED is a III-V semiconductor device andthe re-emitting semiconductor construction is a II-VI semiconductordevice.

In one embodiment of the present invention, the compositions of thevarious layers of the components of the adapted LED are selected inlight of the following considerations. Each layer typically will bepseudomorphic to the substrate at the thickness given for that layer orlattice matched to the substrate. Alternately, each layer may bepseudomorphic or lattice matched to immediately adjacent layers.Potential well layer materials and thicknesses are typically chosen soas to provide a desired transition energy, which will correspond to thewavelength of light to be emitted from the quantum well. For example,the points labeled 460 nm, 540 nm and 630 nm in FIG. 3 representCd(Mg)ZnSe alloys having lattice constants close to that for an InPsubstrate (5.8687 Angstroms or 0.58687 nm) and band gap energiescorresponding to wavelengths of 460 nm (blue), 540 nm (green) and 630 nm(red). Where a potential well layer is sufficiently thin thatquantization raises the transition energy above the bulk band gap energyin the well, the potential well may be regarded as a quantum well. Thethickness of each quantum well layer will determine the amount ofquantization energy in the quantum well, which is added to the bulk bandgap energy to determine the transition energy in the quantum well. Thus,the wavelength associated with each quantum well can be tuned byadjustment of the quantum well layer thickness. Typically thicknessesfor quantum well layers are between 1 nm and 100 nm, more typicallybetween 2 nm and 35 nm. Typically the quantization energy translatesinto a reduction in wavelength of 20 to 50 nm relative to that expectedon the basis of the band gap energy alone. Strain in the emitting layermay also change the transition energy for potential wells and quantumwells, including the strain resulting from the imperfect match oflattice constants between pseudomorphic layers.

Techniques for calculating the transition energy of a strained orunstrained potential well or quantum well are known in the art, e.g., inHerbert Kroemer, Quantum Mechanics for Engineering, Materials Scienceand Applied Physics (Prentice Hall, Englewood Cliffs, N.J., 1994) at pp.54-63; and Zory, ed., Quantum Well Lasers (Academic Press, San Diego,Calif., 1993) at pp. 72-79; both incorporated herein by reference.

Any suitable emission wavelengths may be chosen, including those in theinfrared, visible, and ultraviolet bands. In one embodiment of thepresent invention, the emission wavelengths are chosen so that thecombined output of light emitted by the adapted LED creates theappearance of any color that can be generated by the combination of two,three or more monochromatic light sources, including white or near-whitecolors, pastel colors, magenta, cyan, and the like. In anotherembodiment, the adapted LED according to the present invention emitslight at an invisible infrared or ultraviolet wavelength and at avisible wavelength as an indication that the device is in operation.Typically the short-wavelength LED emits photons of the shortestwavelength, so that photons emitted from the short-wavelength LED havesufficient energy to drive the potential wells in the re-emittingsemiconductor construction.

FIG. 1 is a schematic diagram of an adapted LED according to oneembodiment of the present invention. Adapted LED 50 includesshort-wavelength LED 20 and re-emitting semiconductor construction 10.Re-emitting semiconductor construction 10 may be attached to theemitting surface of short-wavelength LED 20 by any suitable method,including the use of adhesive or welding materials, pressure, heat orcombinations thereof. In the depicted embodiment, adapted LED 50 isflip-chip mounted on header 40. Solder contacts 27 and 28 maintainelectrical contact between LED electrodes 25 and 26 and header traces 42and 43, respectively. Short-wavelength LED 20 is typically a UVwavelength LED or a visible wavelength LED. Where short-wavelength LED20 is a visible wavelength LED, it typically a green, blue or violetwavelength LED and most typically a blue or violet wavelength LED.Short-wavelength LED 20 may comprise any suitable components. In thedepicted embodiment, short-wavelength LED 20 comprises electricalcontacts 25 and 26, a transparent base layer 21, and functional layers22, 23 and 24. Functional layers 22, 23 and 24 may represent anysuitable LED construction but typically represent a pn junction,including p- and n-doped semiconductors 22 and 24 and a light-emittingregion 23 which may comprise one or more quantum wells. A re-emittingsemiconductor construction 10 according to the present invention ismounted on the emitting surface of the short-wavelength LED 20. In thedepicted embodiment, re-emitting semiconductor construction 10 comprisesred quantum well layer 12, green quantum well layer 14, and intermediatelayers 11, 13 and 15. In one embodiment of the present invention,intermediate layers 11, 13 and 15 include support layers and absorbinglayers, as described below. In one typical embodiment, short wavelengthLED 20 is a III-V semiconductor device, such as a blue-emittingGaN-based LED, and re-emitting semiconductor construction 10 is a II-VIsemiconductor device.

FIG. 2 is a band diagram representing conduction and valence bands ofsemiconductors in a re-emitting semiconductor construction according toone embodiment of the present invention. Layer thickness is notrepresented to scale. Table I indicates the composition of layers 1-9 inthis embodiment and the band gap energy (E_(g)) for that composition.This construction may be grown on an InP substrate.

TABLE I Layer Composition Band gap Energy (E_(g)) 1Cd_(0.24)Mg_(0.43)Zn_(0.33)Se 2.9 eV 2 Cd_(0.35)Mg_(0.27)Zn_(0.38)Se 2.6eV 3 Cd_(0.70)Zn_(0.30)Se 1.9 eV 4 Cd_(0.35)Mg_(0.27)Zn_(0.38)Se 2.6 eV5 Cd_(0.24)Mg_(0.43)Zn_(0.33)Se 2.9 eV 6 Cd_(0.35)Mg_(0.27)Zn_(0.38)Se2.6 eV 7 Cd_(0.33)Zn_(0.67)Se 2.3 eV 8 Cd_(0.35)Mg_(0.27)Zn_(0.38)Se 2.6eV 9 Cd_(0.24)Mg_(0.43)Zn_(0.33)Se 2.9 eV

Layer 3 represents a single potential well which is a red-emittingquantum well having a thickness of about 10 nm. Layer 7 represents asingle potential well which is a green-emitting quantum well having athickness of about 10 nm. Layers 2, 4, 6 and 8 represent absorbinglayers, each having a thickness of about 1000 nm. Layers 1, 5 and 9represent support layers. Support layers are typically chosen so as tobe substantially transparent to light emitted from quantum wells 3 and 7and from short-wavelength LED 20. Alternately, the device may comprisemultiple red- or green-emitting potential wells or quantum wellsseparated by absorbing layers and/or support layers.

Without wishing to be bound by theory, it is believed that theembodiment of the present invention depicted in FIG. 1 operatesaccording to the following principles: When an electrical current passesbetween electrodes 25 and 26, short-wavelength photons are emitted fromshort-wavelength LED 20. Photons traveling in the direction of theemitting surface of short-wavelength LED 20 enter re-emittingsemiconductor construction 10. Photons passing through re-emittingsemiconductor construction 10 may be absorbed and re-emitted from thegreen-emitting quantum well 7 as green-wavelength photons or from thered-emitting quantum well 3 as red-wavelength photons. The absorption ofa short-wavelength photon generates an electron-hole pair which may thenrecombine in the quantum wells, with the emission of a photon. Thepolychromatic combination of blue-, green- and red-wavelength lightemitted from the device may appear white or near-white in color. Theintensity of blue-, green- and red-wavelength light emitted from thedevice may be balanced in any suitable manner, including manipulation ofthe number of quantum wells of each type, the use of filters orreflective layers, and manipulation of the thickness and composition ofabsorbing layers. FIG. 4 represents a spectrum of light that emits fromone embodiment of the device according to the present invention.

Again with reference to the embodiment represented by FIG. 2, absorbinglayers 2, 4, 5 and 8 may be adapted to absorb photons emitted from shortwavelength LED 20 by selecting a band gap energy for the absorbinglayers that is intermediate between the energy of photons emitted fromshort wavelength LED 20 and the transition energies of quantum wells 3and 7. Electron-hole pairs generated by absorption of photons in theabsorbing layers 2, 4, 6 and 8 are typically captured by the quantumwells 3 and 7 before recombining with concomitant emission of a photon.Absorbing layers may optionally have a gradient in composition over allor a portion of their thickness, so as to funnel or direct electronsand/or holes toward potential wells.

Where the short-wavelength LED 20 is a visible wavelength LED, layers11-15 of re-emitting semiconductor construction 10 may be partiallytransparent to the light emitted from the short wavelength LED. Vector Brepresents a blue wavelength photon passing through re-emittingsemiconductor construction 10. Vector R represents a red wavelengthphoton emitted from red quantum well layer 12 after absorption of a bluewavelength photon emitted from short-wavelength LED 20. Vector Grepresents a green wavelength photon emitted from green quantum welllayer 14 after absorption of a blue wavelength photon emitted fromshort-wavelength LED 20. Alternately, where short-wavelength LED 20 is aUV wavelength LED, layers 11-15 of re-emitting semiconductorconstruction 10 may block a greater portion or substantially orcompletely all of the light emitted from the short wavelength LED 20, sothat a greater portion or substantially or completely all of the lightemitted from the adapted LED 50 is light re-emitted from re-emittingsemiconductor construction 10. Where short-wavelength LED 20 is a UVwavelength LED, re-emitting semiconductor construction 10 may includered-, green- and blue-emitting quantum wells.

The adapted LED according to the present invention may compriseadditional layers of conducting, semiconducting or non-conductingmaterials. Electrical contact layers may be added to provide a path forsupply of electrical current to the short-wavelength LED. Electricalcontact layers may be placed such that the current passes also throughthe re-emitting semiconductor construction, or such that the currentdoes not pass through the re-emitting semiconductor construction. Lightfiltering layers may be added to alter or correct the balance of lightwavelengths in the light emitted by the adapted LED. To improvebrightness and efficiency, layers comprising a mirror or reflector maybe added.

In one embodiment, the adapted LED according to the present invention isa white or near-white LED which emits light at four principalwavelengths in the blue, green, yellow and red bands. In one embodiment,the adapted LED according to the present invention is a white ornear-white LED which emits light at two principal wavelengths in theblue and yellow bands.

The adapted LED according to the present invention may compriseadditional semiconductor elements comprising active or passivecomponents such as resistors, diodes, zener diodes, conventional LED's,capacitors, transistors, bipolar transistors, FET transistors, MOSFETtransistors, insulated gate bipolar transistors, phototransistors,photodetectors, SCR's, thyristors, triacs, voltage regulators, and othercircuit elements. The adapted LED according to the present invention maycomprise an integrated circuit. The adapted LED according to the presentinvention may comprise a display panel or an illumination panel.

The short-wavelength LED and the re-emitting semiconductor constructionwhich make up the adapted LED according to the present invention may bemanufactured by any suitable method, which may include molecular beamepitaxy (MBE), chemical vapor deposition, liquid phase epitaxy and vaporphase epitaxy. The elements of the adapted LED according to the presentinvention may include a substrate. Any suitable substrate may be used inthe practice of the present invention. Typical substrate materialsinclude Si, Ge, GaAs, IP, sapphire, SiC and ZnSe. The substrate may ben-doped, p-doped, or semi-insulating, which may be achieved by anysuitable method or by inclusion of any suitable dopant. Alternately, theelements of the adapted LED according to the present invention may bewithout a substrate. In one embodiment, elements of the adapted LEDaccording to the present invention may be formed on a substrate and thenseparated from the substrate. The elements of the adapted LED accordingto the present invention may be joined together by any suitable method,including the use of adhesive or welding materials, pressure, heat orcombinations thereof. In one embodiment, the re-emitting semiconductorconstruction is formed on a substrate, bonded to the short-wavelengthLED, and then its substrate is removed by physical, chemical orenergetic methods. Typically, the bond created is transparent. Bondingmethods may include interfacial or edge bonding. Optionally, refractiveindex matching layers or interstitial spaces may be included.

The adapted LED according to the present invention may be a component orthe critical component of a graphic display device such as a large- orsmall-screen video monitor, computer monitor or display, television,telephone device or telephone device display, personal digital assistantor personal digital assistant display, pager or pager display,calculator or calculator display, game or game display, toy or toydisplay, large or small appliance or large or small appliance display,automotive dashboard or automotive dashboard display, automotiveinterior or automotive interior display, marine dashboard or marinedashboard display, marine interior or marine interior display,aeronautic dashboard or aeronautic dashboard display, aeronauticinterior or aeronautic interior display, traffic control device ortraffic control device display, advertising display, advertising sign,or the like.

The adapted LED according to the present invention may be a component orthe critical component of a liquid crystal display (LCD), or likedisplay, as a backlight to that display. In one embodiment, thesemiconductor device according to the present invention is speciallyadapted for use a backlight for a liquid crystal display by matching thecolors emitted by the semiconductor device according to the presentinvention to the color filters of the LCD display.

The adapted LED according to the present invention may be a component orthe critical component of an illumination device such as a free-standingor built-in lighting fixture or lamp, landscape or architecturalillumination fixture, hand-held or vehicle-mounted lamp, automotiveheadlight or taillight, automotive interior illumination fixture,automotive or non-automotive signaling device, road illumination device,traffic control signaling device, marine lamp or signaling device orinterior illumination fixture, aeronautic lamp or signaling device orinterior illumination fixture, large or small appliance or large orsmall appliance lamp, or the like; or any device or component used as asource of infrared, visible, or ultraviolet radiation.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

1. An adapted LED comprising a UV LED and a re-emitting semiconductorconstruction, wherein said re-emitting semiconductor constructioncomprises at least one first potential well not located within a pnjunction having a first transition energy corresponding toblue-wavelength light, at least one second potential well not locatedwithin a pn junction having a sccond transition energy corresponding togreen-wavelength light, and at least one third potential well notlocated within a pn junction having a third transition energycorresponding to red-wavelength light, wherein said at least one firstpotential well includes a first quantum well, wherein said at least onesecond potential well includes a second quantum well, and wherein saidat Icast one third potential well includes a third quantum well, there-emitting semiconductor construction additionally comprising anabsorbing layer closely adjacent to and having a band gap energy greaterthan a transition energy of a potential well within the re-emittingsemiconductor construction.
 2. An adapted LED comprising a UV LED and are-emitting semiconductor construction, wherein said re-emittingsemiconductor construction comprises at least one first potential wellnot located within a pn junction having a first transition energycorresponding to blue-wavelength light and at least one second potentialwell not located within a pn junction having a second transition energycorresponding to yellow-wavelength light, the re-emitting semiconductorconstruction additionally comprising an absorbing layer closely adjacentto and having a band gap energy greater than a transition energy of apotential well within the re-emitting semiconductor construction.
 3. Anadaptcd LED comprising an LED and a re-emitting semiconductorconstruction, wherein the re-emitting semiconductor constructioncomprises a first potential well having a first transition energy and anabsorbing layer closely adjacent to the first potential well, theabsorbing layer having a band gap energy greater than the firsttransition energy, and wherein the re-emitting semiconductorconstruction comprises a second potential well having a secondtransition energy not equal to the first transition energy.